Oriented para-aramid fibers typified by high strength, high modulus, poly(p-phenylene terephthalamide) fiber show little or no change in tenacity when heated while relaxed or under tension at temperatures of 300.degree.-350.degree. C. Relaxed heating at higher temperatures causes a drop in tenacity. Some oriented polyamide fiber such as poly-1,4-benzamide fiber do strengthen when heated under tension by virtue of a crystallization process which improves molecular orientation significantly. Non-para-aramids such as poly(m-phenylene isophthalamide) will show no change or actually decrease in tenacity on heating whether under tension or relaxed, depending on temperature.
In each situation described above, the maximum fiber tenacity is ordinarily attained before the fiber is incorporated into a fabric or other article. Since the present invention deals with tension-free heat-strengthening of aramid fiber and fibrids, it permits the incorporation of fiber into a fabric or fibrids into a paper and subsequent heat treatment to achieve greater strength.
It is believed that the placement of the substituents in the polymer of the as-spun filaments enables the filaments to be heat-strengthened while closely related materials will not survive the heat treatment of the invention. For example, ##STR1## which illustrates thermal cyclization to a class of polymers known as polybenzoxazinones ("Encyclopedia of Polymer Science and Technology" V. 10 pp 682-690, Interscience, N.Y., 1969). In the present invention, this reaction cannot take place, although some dehydration to anhydride may occur upon heat treatment. However, such dehydration does not affect capability of the polymer to form a covalent bond with epoxides and the like.